Enigmas of the sky: partners or strangers?

Enigmas of the sky: Partners or strangers?

Two astronomers report a possible and controversial physical link between two of the more dramatic heavenly enigmas: the brightest quasar ever identified and a recently discovered hydrogen cloud taht has variously been characterized as a newborn galaxy or an already evolved irregular dwarf galaxy.

The potential link hinges on the object's alighment and a long-running astronomical controversy. No one disputes that the quasar 3C273, as viewed from Earth, appears to lie near the hydrogen cloud in the Virgo cluster, a regoin of energetic galaxies. Researchers also agree that a radio jet from the quasar appears to line up almost exactly with the cloud's center. For Halton C. Arp of the Max Planck Institute for Physics and Astrophysics in Garching, West Germany, and Geoffrey Burgidge of the University of California, San Diego, that's enough to assume a possible physical association between the two ojects -- even though the quasar has a redshift some 40 times greater than the Virgo cloud, suggesting a vast distance separates the two.

In the usual interpretation of cosmological redshift -- a well-known effect in which light from a rapidly receding source appears redder than it would had it remained at rest -- the greater the shift, the greater the object's distance from an observer. By that standard, which fits the widely accepted model of an expanding universe, 3C273 should lie several billion light years in back of the cloud, and no physical link between the two would seem possible.

But Arp and Burbidge, citing their previous arguments that larger redshifts do not always signify greater distances, contend the quasar and cloud lie astronomically close to one another. "These two objects are rare, and it seems to me that their alignment is more than just a statistical argument; they really are physically associated," says Burbidge.

In the April 10 ASTROPHYSICAL JOURNAL LETTERS, Arp and Burbidge suggest several scenarious for the proposed link. They note that the quasar jet might have spewed the hydrogen cloud from 3C273. A more likely association, they say, is that radiation from the quasar has ionized the intergalactic medium of the Virgo cluster, exciting atomic hydrogen gas (which would not normally emit radiation) to produce the radio emissions that led to the hydrogen cloud's discovery last September (SN: 9/15/89, p. 164).

"While it's an interesting [proposed] connection between the cloud and the quasar," ignoring redshift as a measure of distance "means you first have to overthrow all the evidence that supports taht view," notes astronomer Cyril Hazard of the University of Pittsburgh.

Astronomer Arthur M. Wolfe at the University of California, San Diego, suggests a way to test Arp and Burbidge's idea. If the Virgo cloud and the quasar were negihbors, then 3C273 would heat the cloud, causing it to absorb far less 21-centimeter radio emission than it normally would from a distant quasar, Wolfe told SCIENCE NEWS.

Michael J. Irwin of the University of Cambridge in England asserts that the cloud is not as rare an object as originally thought, and so does not require a quasar to explain its existence or its radiation emission. Wolfe and Richard G. McMahon of Cambridge announced last September that the presumed starless hydrogen cloud in Virgo actaully contains some starlight and may harbor an irregular dwarf galaxy. Irwin, along with Wolfe, Hazard, McMahon and the cloud's discoverers -- Martha P. Haynes of Cornell University and Riccardo Giovanelli of the Arecibo Observatory in Puerto Rico -- expect to report details of the starlight findings in the August 1990 ASTROPHYSICAL JOURNAL.

Irwin told SCIENCE NEWS that a colleague's recent redshift measurements indicate the purported dwarf galaxy moves with the cloud, suggesting the two are physically associated.